Anatomy and Biomechanics of the Glenohumeral Capsule

The shoulder is an inherently loose articulation permitting a wide range of motion. The glenohumeral capsule remains lax throughout mid-range motion where stability is conferred by the dynamic action of the rotator cuff and the conforming articulation of the humeral head and the glenolabral surface. The capsuloligamentous structures about the glenohumeral joint act as passive restraints when capsular tension develops at the extremes of motion and contributes to glenohumeral stability.

Each portion of the joint capsule performs its primary “checkrein” responsibility at different glenohumeral positions. The rotator interval develops tension in external rotation with the arm adducted. The middle glenohumeral ligament restricts motion with the arm in external rotation in the midrange of abduction, while the anterior band of the inferior glenohumeral ligament stabilizes the glenohumeral joint in full abduction and external rotation.

Tension is developed in the posterior-superior capsule with the arm adducted and internally rotated. This tension shifts posterior-inferior with abduction and internal rotation. Contracture of any given portion of the glenohumeral capsule will cause reduction in glenohumeral motion as though the end range of motion were reached prematurely.

In addition, some capsular contractures cause an obligate and abnormal translation of the humeral head during motion. The abnormal joint mechanics associated with posterior capsular contracture have been observed to result in superior migration, which can be associated with subacromial impingent or superior labral tearing .

Chronic anterior asymmetric tightness may cause posterior translation with increased posterior glenohumeral joint reaction forces resulting in posterior glenoid wear. Normal shoulder range of motion requires a smooth glenohumeral articulation with a surrounding capsule of normal volume and compliance, unrestricted gliding of the rotator cuff tendons under the coraco-acromial arch and under the deep surface of the deltoid muscle, and normal translation of the scapula over the chest wall. Adhesions or contracture in any of these locations may result in shoulder stiffness.